Rename playbook variables so they are consistently prefixed

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

roles/matrix-server/tasks/import_sqlite_db.yml

@@ -52,7 +52,7 @@
 - name: Importing SQLite database into Postgres
   docker_container:
     name: matrix-synapse-migrate
-    image: "{{ matrix_docker_image_synapse }}"
+    image: "{{ matrix_synapse_docker_image }}"
     detach: no
     cleanup: yes
     entrypoint: /usr/bin/python